This invention relates to improvements in a control means for closing the damper for a furnace flue when the associated furnace is not burning. Many automatic controls for such a damper have been proposed for the purpose of conserving fuel by closing the flue to prevent heat loss therethrough when the furnace is not operating. A characteristic common to many such controls is a mechanical or electrical linkage between the actuating means of the control and the actuated means connected with the damper.
Other types of controls operate to open a normally closed damper when the furnace is burning. In either type of control, a failure in the damper operating linkage could leave the flue closed when the furnace is burning, with the result that dangerous furnace combustion products could escape into a living area. A similar objectional consequence could occur with many such controls in the event of power failure for the damper control.
The following patents are typical of the prior art: Scott U.S. Pat. No. 1,743,731 shows a furnace blower that causes a normally closed damper (in contrast to applicant's normally open damper) to open when the furnace is burning. Goodridge U.S. Pat. No. 1,863,443 discusses the possibility of using a positive displacement blower to close the flue when the furnace is not operating. Otherwise no automatic damper control is shown. Wetzsteon U.S. Pat. No. 2,296,410 is concerned with control means for effecting a substantially constant flue draft pressure. No means are shown for closing the damper when the furnace is not burning. The above noted patents, as well as Olsen U.S. Pat. No. 2,303,135 and Holtzman et al U.S. Pat. No. 3,273,625 show various elements per se suitable for use in applicant's construction, but in entirely unrelated applications.